1. Field of the Invention
This invention relates to the field of industrial robots, more particularly to the wrist configuration for a robot arm and especially to a nonsingular wrist construction.
2. Description of the Prior Art
Most wrist mechanisms used with industrial robot arms exhibit certain singular positions at which articulation becomes limited, i.e., where one degree of freedom is lost. Normally, the singularities occur well within the normal operating range of the wrist.
Increasingly, commercial robots are required to traverse a complicated three dimensional path at high speed and within a stringent accuracy tolerance. In these conditions, a robot having six degrees of freedom is needed to accomplish the desired motion. The motion of a tool carried by the robot arm forms a continuous path both in position and orientation. As wrist agility and dexterity increase, the smoothness of the path traversed by the tool and its coverage increase accordingly. Wrist agility is usually determined by its roll speed, i.e., the speed of rotation about the roll axis. Wrist dexterity is determined by the range of bend-back angle and the range of roll rotation. The manipulator for an industrial robot is generally moveable with respect to a reference base, such as a robot arm or boom, through a multiple axis articulated joint system through which the base and manipulator are connected. Power supplied to a component of the arm is transmitted through gearing to the manipulator, which grips a tool or another workpiece. Examples of systems which have employed bevel gear trains for this purpose are described in U.S. Pat. Nos. 4,353,677 and 4,365,928.
The robot arm on which the manipulator is attached can either be an inflexible structure having a wrist mechanism at its end, which can accommodate five or six degrees of freedom of the manipulator, or the robot arm itself may be flexible and comprise successive links hinged to each other by means of universal joints in order to produce the required translation and rotary movement of the manipulator. U.S. Pat. No. 4,107,948 describes a robot arm of this type. Another example of an articulating mechanism that produces large angular rotation of the manipulator with respect to the base using a multiple link connected in series along the arm is described in U.S. Pat. No. 3,580,099.
Also known in the prior art are mechanisms for transmitting along the arm axial motion applied to the base or inner end of the manipulator and converting that motion to articulation of the outer end of the manipulation arm. In the example described in U.S. Pat. No. 4,300,362, two rings spaced axially along the arm are linked by two connecting rods that are connected to the ring by joints having two degrees of freedom. In order to control the displacement of the outer ring relative to the inner ring, two control rods apply force through an elbow attached to the inner ring, the elbow having two degrees of freedom and being connected to links that extend transversely along the axis of the manipulating arm.
Complete control of tool orientation requires three rotational degrees of freedom; roll, i.e., rotation of the tool about its longitudinal centerline; pitch, vertical deflection of the tool centerline; and yaw, lateral deflection of the tool centerline. In a conventional wrist mechanism known from the prior art, the first link is rotatably connected to the robot arm. The second link is rotatably connected to the first link, and the axis of the second rotation is usually perpendicular to the first axis. A third wrist link is rotatably connected to the second link, and the axis of the third rotation is usually perpendicular to the second link. The angle between the first and the third axes is not constant but varies depending on the angular displacement of the second axis. Singularity occurs when the first and third axes become parallel or coincident, a condition called "gimbal lock".
For example, if two shafts are connected by an ordinary universal joint such as Hooke joint or Cardan joint, the shafts cannot rotate when the deflection angle of the joint becomes 90 degrees. Nonsingular wrist mechanisms in the prior art avoid gimbal lock by providing additional rotary joints. The total angular deflection of the end link and tool is distributed over several joints, each joint articulating through substantially less than 90 degrees. By positive mechanical means, the deflection angle of consecutive joints are constrained to depend on each other so that the overall mechanism possesses no more than three degrees of freedom. Usually three or four universal joints in series are interposed between the first and the end wrist links. A disadvantage of such wrists is a large amount of backlash and the associated lack of precision and stiffness required to control within acceptable tolerance the orientation of the end link and tool.